It’s commonly recommended that sunflowers should always be direct-sown. While you do get taller, stronger plants from directly sowing sunflowers, you can start them indoors and transplant them outside later. The main benefits of starting sunflowers indoors is to get a slight head start on the growing season and to avoid pests from eating your seeds and tender sprouts.
Advantages of Transplanting Sunflowers
Starting your sunflowers indoors allows you to get a slight head start if you have a short growing season, and also allows your sunflowers to stay protected from pests that would otherwise dig up your seeds or eat the tender seedlings.
Some sunflowers, especially the large-headed varieties, take a long time to flower and mature. Giving your sunflowers a few weeks head start can ensure you get your blooms earlier in the summer instead of late summer. A few weeks won’t make a difference in most climates, but for northern gardeners growing sunflowers for seeds, it can help guarantee a harvest.
Pests are the bane of direct-sown sunflowers. Just as we enjoy sunflower seeds or tender sunflower sprouts, so do many other critters. Field mice, chipmunks, squirrels and some birds can dig up and eat your sunflower seeds. And once they sprout, they’re vulnerable to rabbits, slugs, and snails.
Sunflowers rapidly start growing once the first true leaves appear (the leaves that appear after the first two seed leaves), so starting your sunflower seeds indoors will give them a better chance to survive the most vulnerable stage of development.
Disadvantage of Transplanting Sunflowers
The disadvantage of transplanting sunflowers is that they may end up being stunted, especially if your transplants are rootbound, and may also require staking. When sown directly outside, sunflowers have a deep taproot and wide network of roots that support larger, stronger plants. Transplanting can disturb or damage the roots, which means your sunflowers can end up being shorter than you expect. In addition, since the roots will not be as deep, your sunflowers may require staking.
However, if you transplant early, before your sunflower seedlings get rootbound, you can minimize stunting. As a rule of thumb, transplant out no later than when the first true leaves appear, and always be careful not to damage the roots while transplanting.
Pro-Tip: Some gardeners purposely let their sunflower seedlings get rootbound before transplanting so they can have shorter, more manageable plants.
When to Direct Sow Sunflowers Outside
Sow your sunflower seeds outside after the threat of frost has passed, and ideally when soil temperatures stay above 50°F (10°C). Sunflowers are frost tender, and young seedlings may not survive a light frost.
Depending on your climate, that could be anywhere from 2 weeks to 4 weeks after your average last frost date. You can sow when temperatures are colder than 50°F, but germination and initial growth will be slower.
When to Start Sunflower Seeds Indoors
To minimize post-transplant stunting, sow indoors 1 to 2 weeks before your expected last frost date. This will give you enough time to sprout your sunflowers, and by the time they’re ready to transplant, any threat of a spring frost should be gone.
When and How to Transplant Sunflowers
Transplant sunflowers after the threat of frost has passed, ideally no later than when the first true leaves appear.
When transplanting sunflowers, space them the same way you would direct-sown sunflowers. For rows of tall, large-headed sunflowers, the ideal spacing is 3 feet (1 meter) apart, however, you can plant them closer together without. For small sunflowers, or a few large sunflowers planted together, you can get away with 1 foot (30 cm) of space between them without much crowding.
Transplant sunflowers the way you would cucumbers, pumpkins, or squash, being very careful not to damage the roots. Sunflowers can also benefit from a small amount of organic, slow-release fertilizer mixed into the planting hole before transplanting. After transplanting, water it well to reduce transplant shock and settle the soil around the roots.
Where to Transplant Sunflowers
Sunflowers love full sun, at least 6 hours of unfiltered direct sunlight per day. Therefore, anywhere they can get full sun is strongly recommended. Along the south side of a wall, fence, or deck/patio are examples of ideal locations.
Note: Consider how your sunflowers will affect the plants around them when direct sowing or transplanting. For instance, if you plant a row of tall Titan sunflowers along the south side of a vegetable bed, your sunflowers will get full sun, but once they’re fully grown, they’ll completely shade out the vegetables behind them.
Related Questions About Transplanting Sunflowers
Can You Transplant Large Sunflower Plants?
You can transplant a large sunflower from a smaller pot to a larger pot, or transplant a large potted sunflower into the ground. After some transplant shock, the plant will stay healthy, but the plant will not continue to grow much taller especially if it already has buds and flowers. It may also require staking, depending how tall it is.
However, it’s not recommended to transplant fully grown sunflowers from the ground to another location, as sunflower roots can grow several feet deep, and a smaller, damaged root ball will not be able to support a fully grown sunflower plant, leading to severe transplant shock.
Do I Need Grow Lights to Start Sunflowers Indoors?
Since sunflower seedlings won’t grow indoors longer than a few weeks, you can start sunflowers on a sunny, south-facing windowsill (windows facing other directions will cause your seedlings to be too leggy). However, if you have a fluorescent or LED grow light, that will promote stronger, less leggy seedlings.